Aircraft tire treads, in contrast to more conventional vehicular tire treads, are subjected to extreme operating conditions which require the tire treads to endure significant forces experienced upon landing of an aircraft as the tire touches ground and instantly accelerates to relatively high speeds under load as well as significant forces experienced upon aircraft takeoffs which require a rapid acceleration to relatively high speeds under load.
In addition, aircraft tires, in contrast to more conventional vehicular tires, are often significantly stiffer in nature as a result of, at least in part, often being composed of a significant plurality of carcass plies, which for some aircraft tires may be as many as 8 or more plies, and as a result tends to be a significantly more hysteretic tire to thereby have a greater propensity for internal heat generation with a resultant greater temperature rise during the working of the tire which impacts negatively upon the heat durability of the tire.
The aforesaid extreme landing and take-off operational conditions for the relatively stiff, relatively hysteretic aircraft tires inherently create a significant rapid temperature rise for the aircraft tread due to internal heat generation for which long term heat durability of the tread rubber composition may be of concern as well as submitting the running surface of the tire tread to significant abrasion forces for which long term tread wear may be of a concern.
In practice, such aircraft tire treads are conventionally composed of a relatively tough, abrasion resistant, natural rubber (and sometimes a minor amount of polybutadiene rubber for the abrasion resistance promoting aspect of the tire tread) based rubber composition which is relatively hysteretic in nature and therefore prone to internal heat generation caused by the aforesaid operationally exerted forces.
A typical significant elastomeric component for such aircraft tread rubber compositions to promote abrasion resistance is the relatively minor amount of cis 1,4-polybutadiene rubber contained in a predominately natural rubber tread rubber composition where the cis 1,4-polybutadiene rubber typically has a moderate number average molecular weight (Mn) in a range of from about 175,000 to about 275,000 and a weight average molecular weight (Mw) in a range of from about 400,000 to about 650,000 with a relatively narrow (low) heterogeneity index (Mw/Mn) in a range of from about 1.5/1 to about 2.5/1.
In practice, the needed abrasion resistance for such aircraft tire tread is typically obtained by the inclusion of the minor amount of the cis 1,4-polybutadiene rubber of moderate molecular weight (to promote abrasion resistance) together with a relatively high loading of rubber reinforcing carbon black (to also promote abrasion resistance).
It is desired herein to provide an aircraft tire tread composition which, in its unvulcanized state can be suitably processed in conventional rubber processing equipment, including acceptably extruded into relatively smooth surfaced tread strips, for which a major portion of the elastomer component of the tread rubber composition is a cis 1,4-polybutadiene rubber (which would ordinarily be expected to have a negative effect upon the rubber composition's processability) to promote resistance to abrasion in the rubber composition's vulcanized state without resorting to the aforesaid high carbon black loading for abrasion resistance which significantly increases internal heat generation and a resultant increase in rate of temperature rise within the tire.
This is considered herein to be a significant challenge, with unspecified alternative adjustments to be made for the aircraft tire tread rubber composition, since the increase in a cis 1,4-butadiene rubber content, and associated reduction in natural rubber content, of the aircraft tire tread rubber composition, would ordinarily be expected to have a negative impact upon the aforesaid processability of the unvulcanized rubber composition, thereby making it more difficult to fabricate (by an extrusion or calendering process) the associated unvulcanized tread component having a relatively smooth surface.
It is well appreciated by those having skill in such art that significant compromises of physical properties of a tire tread rubber composition are sometimes made for various purposes. For example, optimizing an aircraft tire tread rubber composition's increased abrasion resistance by merely increasing its cis 1,4-polybutadiene rubber content may result in relatively disadvantageous results in one or more other desirable properties of an aircraft tire tread rubber composition, including, for example, its aforesaid processability in its unvulcanized state.
FIG. 2 of the included drawings is provided in a form of an outer and inner pentagraph to illustrate, in a pictorial manner, what are considered herein as being five significant physical properties (unvulcanized and vulcanized properties) of an aircraft tire tread rubber composition.
The five included points of the outer pentagraph representing such properties are, in a counterclockwise direction from the top: Hysteresis (hot 100° C. rebound and tangent delta properties), Treadwear (abrasion resistance), Stiffness (tensile and dynamic shear modulus), Tear Resistance (resistance to tear propagation) and Processability (e.g. smooth extrudates of the unvuilcanized rubber composition).
The individual arrows extending from the inner pentagraph to each of the five points of the outer pentagraph are intended to illustrate a desirable, but typically unrealistic, goal of equally enhancing all of such five physical properties for an aircraft tire tread rubber composition.
It is well known to those having skill in the pertinent art that optimizing one of such properties typically detracts, or has a negative impact, in one or more of the other physical properties.
For example, significantly increasing the cis 1,4-polybutadiene rubber in a natural rubber/polybutadiene aircraft tire rubber tread composition would be expected to have a negative impact on the unvulcanized rubber composition's processability although it may have a beneficial effect on its abrasion resistance.
For example, use of reinforcing filler as a combination of precipitated silica and medium particle sized rubber reinforcing carbon black, instead of carbon black alone, is provided to promote a beneficial reduction in the rubber composition's hysteresis (reduction in internal heat build up) and increase in its tear strength. However, such inclusion of the precipitated silica, particularly with a corresponding reduction in the carbon black, would be expected to have a negative impact upon the rubber composition's abrasion resistance. This facilitates the need for a high cis 1,4-polybutadiene rubber to retain abrasion resistance due to the use of silica and medium sized carbon black.
Therefore, a challenge typically remains to provide a suitably processable (in its unvulcanized state), aircraft tire tread rubber composition in which a major portion of its rubber component is a cis 1,4-polybutadiene rubber which promotes (in its vulcanized state) a combination of both suitable abrasion resistance (e.g. DIN abrasion) and hysteresis (100° C. rebound) physical properties, all of which is considered herein to be a departure from past practice for an aircraft tire tread.
For this invention, a focus is on enhancing the abrasion resistance (e.g. DIN abrasion) and hysteresis (e.g. 100° C. rebound property and tan delta property) of an aircraft tire tread rubber composition while also providing the rubber composition in a form of a suitably processable unvulcanized rubber composition in a sense of providing a relatively smooth surfaced extrudate in a form of an unvulcanized rubber tread component (e.g. tread strip) with a minimization of surface defects for building, molding and curing a tire assembly to form an aircraft tire.
The term “running surface” of the aircraft tire, unless otherwise indicated, means the outer surface of the tread which is intended to be ground-contacting.
In the description of this invention, the terms “rubber” and “elastomer” where used herein, are used interchangeably, unless otherwise provided. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art. The terms “cure” and “vulcanize” are well understood by those having skill in such art and may be used interchangeably unless otherwise provided. In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer.
The number average molecular weight (Mn) and weight average molecular weight (Mw) of a cis 1,4-polybutadiene elastomer as referenced herein can be suitably determined by gel permeation chromotography (GPC), a method well known to those having skill in such analytical art.
The heterogeneity index of an elastomer (e.g. a cis 1,4-polybutadiene elastomer) is a ratio of its weight average molecular weight (Mw) to its number average molecular weight (Mn), or Mw/Mn. A relatively low heterogeneity index in a range of from about 1.5/1 to about 2.5/1 is indicative of a relatively narrow molecular weight distribution. A greater heterogeneity index in a range of from about 3/1 to about 5/1, indicating a significantly wider disparity between its weight average molecular weight (Mw) and its number average molecular weight (Mn) is indicative of a relatively broad molecular weight distribution.